206 



ECHINODERMATA ASTEROZOA 



SUB-BRANCH II 



plates. Each of the arms is traversed on the under or oral side by a broad 

 and deep furrow, which tapers gradually in passing from the mouth to the 

 tip of the arm, where it is terminated by a simple grooved plate (Fig. 333) 

 called the ocular plate. The roof of each ambulacral furrow is formed by two 



rows of rafter-like, rather elongate, ambulacral 

 ossicles, the inner ends of which are held 

 together by muscles (Figs. 334, 335). Run- 

 ning along the centre of the groove on its 

 ventral side are placed in succession the radial 

 water-tube, blood-vessel, and nerve cord. 

 tagonaster (?)"' from These are all homologous with the like- 

 Itoeiteg 6 ." ?/r " f named organs of Ophiuroids. 



The form of the ambulacral ossicles 

 varies in different genera. In all recent forms the ends are directly apposed 

 against one another in the median line of the ambulacral grooves ; but in 

 Palaeozoic forms they are arranged in alternate rows, and are inclined towards 

 one another at a very small angle. Each pair of ambulacral plates is ex- 

 cavated at the sides, so as to give rise by their apposition to a series of small 



FIG. 333. 



Ocular plates of 

 Pentagonctter(T) from 

 the Upper Jura of 

 Streitberg. 2/1. 



FIG. 334. 



Detached ambu- 

 lacral ossicle of Pen- 



md- 



Astropecten aurantiaciis, Linn. sp. Recent ; 

 Mediterranean. Enlarged vertical section of 

 one of the arms, am, Ambulacral ossicles ; 

 ad, Adambulacral plate ; mv, Infero-marginal 

 plate ; md, Supero-marginal plate ; i, Super- 

 ambulacral plate. 



Asterias rubens, Linn. Recent ; 

 German Ocean. Enlarged vertical 

 section of one of the arms, am, Am- 

 bulacral ossicles ; ad, Adambulacral 

 plates ; mv, Infero-marginal plates ; 

 a, Radiating water-tube ; &, Ampullae ; 

 p, Tube feet. 



apertures, through which the distensible tube feet or pedicels are emitted, 

 The latter are the downward prolongations of lateral branches given off by 

 the radial ambulacral vessel ; the upward prolongations of the same form 

 small sacs called ampullae, by means of which water is forced into the 

 tube feet. 



The lower ends of the ambulacral ossicles rest against a series of 

 adambulacral plates, and in many forms these are bounded in turn by large 

 marginal plates (Fig. 335). Intermediate plates are those which are inserted 

 between the infero-marginal plates and the adambulacral plates. By the 

 term dorsal plates are understood all calcareous bodies occurring on the dorsal 

 side of the body. 



Perfectly preserved Star-fishes are known only from a few localities, 

 such as Bundenbach in Rhenish Prussia, the usual mode of occurrence being 

 in the form of moulds or detached plates. The earliest forms are found in 

 Cambrian rocks. 



